Areas as far as 15 km (10 mi) from an explosive eruption could be swept by hot,
fast-moving pyroclastic flows and surges. A pyroclastic flow is a ground-
hugging avalanche of hot ash, pumice, rock fragments, and volcanic gas that
rushes down the side of a volcano as fast as 100 km/hour (over 62 mi/hr) or
more. The temperature within a pyroclastic flow may be greater than 500° C
(932° F), sufficient to burn and carbonize wood. Once deposited, the ash,
pumice, and rock fragments may deform (flatten) and weld together because of the
intense heat and the weight of the overlying material.

Scientists examine the distances that pyroclastic flows are known to have
traveled in the past few thousand years to determine areas where they might
occur in the future. The actual areas covered by pyroclastic flows or surges
during a future explosive eruption depends primarily on the location of the
vent(s), the surrounding topography, and volume of magma erupted.

This hazard zone is based on explosive eruptions from vents located along the
chain in the past 10,000 years that are known to have ejected <1 km3
(0.25 mi3) of magma and generated pyroclastic flows or surges. The
zone is centered along the south moat of the caldera, which is the location of
epicenters of many swarms of earthquakes since 1980 and the area of most intense
ground movement (deformation). Thus, scientists suggest that future eruptions
might occur from this restless zone, and pyroclastic flows and surges could
travel as far as 15 km (10 mi) from a new vent. Future pyroclastic flows and
surges from a single eruption in the south moat would affect only a part of the
total hazard zone shown on the map.

If future monitoring of Long Valley volcanic unrest suggests magma is moving
beneath the Mono-Inyo chain, it may not be possible for scientists to anticipate
the exact location of one or more eruptive vents. For example, about 600 years
ago eruptions from both the northern and southern part of the chain occurred
from vents along zones 6-11 km (4-7 mi) in length. Thus, scientists will
probably not be able to define the pyroclastic-flow and -surge zone (left) more
precisely if an eruption is expected.